By this logic, you do not believe anyone because some people have lied. The vast majority of cops do not lie under oath. Take any subset of the population and you will find it’s the same" we’re mostly honest, especially when asked to testify in a court of law. With technology and 24/7 news media, it may be more apparent that some of us have previously managed to get away with lying but cops are not more inclined to lie under oath than any other group of people. Who here hasn’t lied?
